As is well known in the art, cylinder locks generally include a plug arranged for rotation in a lock cylinder housing. Plug pins are slidingly disposed in the plug and are arranged to move against driver pins, which are disposed in bores formed in the cylinder housing and are spring biased toward the axis of the plug rotation. Insertion of a properly cut key in a keyway provided in the plug moves the plug pins against the driver pins and aligns all the pins along a shear line defined by the plug outer circumference, thereby permitting rotation of the plug to cause operation of a latch or locking mechanism.
The combination of key cuts that correctly move all the plug pins to the shear line is commonly referred to as bitting. Lock/key manufacturers typically define the bitting in order to have a large number of possible combinations while still maintaining a secure, functional and durable key. The key cuts have a range of depth, ranging from the shallowest possible cut to the deepest possible cut. Another parameter is the spacing between key cuts, i.e., the distance from the center of one cut to the center of an adjacent cut. Each key cut is designed to move one plug pin at discrete plug pin locations, also referred to as plug pin stations. Each plug pin moves in a discrete bore formed in the plug.
Some keys are made for interacting with telescoping pins. In such a case, each telescoping plug pin has two or more pins that move independently of each other. For telescoping plug pins, the key cuts formed in the key overlap each other to some extent. Each key cut moves a different one of the pins that make up the telescoping pin to the shear line.
Key cuts are typically made by a key cutting or duplicating machine that machines cuts into a key blank. The machining operation is typically done by a cutting tool with sloped sides that cuts into the key blank. The key cut thus has sloping sides. This is also true for tools that stamp the cuts into the key blank.
Lock/key manufacturers typically define a maximum adjacent cut specification (MACS). That is, it is normally not possible to have a large difference in key cut depth between neighboring cut positions. See, for example, U.S. Patent Applications 20090277239 and 20090301144 assigned to Ingersoll-Rand Company, which clearly state that a key cut that violates the MACS is not an available key cut.
The general idea of MACS is explained with reference to FIG. 1; the telescoping case will be explained afterwards with reference to FIG. 1A. A key 1 is shown with two adjacent key cuts 2 and 3 for moving plug pins 4 and 5 of different lengths to a shear line 6. Let us examine what happens if a deeper cut 7 (as shown by the broken line) were to be made instead of key cut 3. Because the key cutting tool has sloping sides, the tool width extends laterally beyond the center position of the deeper cut and removes key material from the adjacent shallower cut. As a result, the plug pin 4, which was meant for the shallower cut, will not sit at the correct depth; rather it will sit deeper than it should (as shown by the broken line 8) because of the material that has been cut away by cutting the adjacent key cut. The plug pin 4 will not be positioned at the shear line 6 but rather at a line 9 (broken line in the drawing) and the plug will not turn.
Another reason for the MACS limitation is to ensure easy insertion or removal of the key. When the key is inserted into the cylinder lock, the plug pins ride up and down the ramps between cuts. If the angle is too steep, the pins can have trouble riding the ramps and the key can get jammed.
Without limitations to the present invention, the MACS may be generally calculated for the above as follows:
  MACS  =            SP      -      CR              DI      ⁡              (                  tan          ⁢                      CA            2                          )                            wherein SP=pin spacing (spacing between plug pins to be operated by the key)        CR=cut root (length of the bottom (“root”) of the key cut)        DI=depth increment        CA=cut angle (angle of the cutting tool head used to create the key cuts)        
Reference is now made to FIG. 1A, which illustrates the MACS for telescoping pins (e.g., inner and outer pins) of a telescoping plug pin of a cylinder lock plug (referred to as the telescoping MACS). Each of the inner and outer pins has a chamfer, that is, a conical tip. This means the shaft of each pin has an outer diameter which is larger than the diameter of the shaft tip.
Although the invention is not limited to this definition, the MACS for a telescoping pin may be calculated as follows:
  MACS  =            ID      -      CR              2      ⁢                          ⁢              DI        ⁡                  (                      tan            ⁢                          CA              2                                )                    
wherein ID=the outside diameter of the inner telescoping pin
The telescoping MACS sets a limit for possible depths of adjacent cuts, and thereby reduces the number of possible key cut combinations.
Efforts have been made in the prior art to increase the MACS for non-telescoping pins. For example, in U.S. Patent Application 20120240646 (corresponding to PCT Application PCT/SE2010/051405), assigned to ASSA OEM AB, Sweden, conical angles of grooves which serve as the key cuts have been changed. In other words, the above formula for MACS still applies; this document simply changes the cut angle CA. All the key cuts are still defined and restricted by the same MACS definition.